The use of electrical measurements in prior art downhole applications, such as logging while drilling (LWD), measurement while drilling (MWD), and wireline logging applications is well known. Such techniques may be utilized to determine a subterranean formation resistivity, which, along with formation porosity measurements, is often used to indicate the presence of hydrocarbons in the formation. For example, it is known in the art that porous formations having a high electrical resistivity often contain hydrocarbons, such as crude oil, while porous formations having a low electrical resistivity are often water saturated. It will be appreciated that the terms resistivity and conductivity, though reciprocal, are often used interchangeably in the art. Mention of one or the other herein is for convenience of description, and is not intended in a limiting sense.
Advances in directional drilling techniques enable the path of a subterranean borehole to be precisely routed in a manner that maximizes oil production. Measurement-while drilling (MWD) and logging-while-drilling (LWD) techniques provide, in substantially real-time, information about a subterranean formation as a borehole is drilled. Such information may be utilized in making steering decisions for subsequent drilling of the borehole. For example, an essentially horizontal section of a borehole may be routed through a thin oil bearing layer. Due to the dips and faults that may occur in the various layers that make up the strata, the drill bit may sporadically exit the thin oil-bearing layer and enter nonproductive zones during drilling. In attempting to steer the drill bit back into the oil-bearing layer, a drilling operator typically needs to know in which direction to turn the drill bit (e.g., up, down, left, or right). In order to make correct steering decisions, information about the strata, such as the dip and strike angles of the boundaries of the oil-bearing layer, is generally useful. Such information may be obtained from azimuthally sensitive measurements of electrical properties (e.g., resistivity) of the surrounding subterranean formation.
Azimuthally sensitive MWD/LWD resistivity tools are disclosed in the prior art and are commonly referred to as directional resistivity tools. While directional resistivity tools have been used commercially, there remains a need for further improvement, and in particular there remains a need for improved antennae for use in directional resistivity logging applications.